mcpctl/docs/reliability.md

Reliability: don't let a bad LLM take mcpctl down

The homelab model changes often (fast ↔ thinking, model swaps, backends that drift or go down). mcpctl must stay responsive and honest through all of it.

Principle

LLM-optional operations must be time-bounded, fall back deterministically, and report the degradation — never hang and never degrade silently.

• Bounded: every optional LLM call is wrapped in withTimeout (Promise.race + an AbortSignal so fetch-based providers actually cancel). A thinking model that streams for minutes can never block the caller.
• Deterministic fallback: when the LLM times out or errors, use the non-LLM path (priority/keyword ordering, byte-range pages).
• Loud, not silent: log the reason ([gate] …, [pagination] …) and tell the user. begin_session prepends ⚠ Smart prompt-selection unavailable (<reason>)… and sets degraded: true + degradedReason on the audit gate_decision event.

Applied in: the gate's begin_session prompt selection (proxymodel/plugins/gate.ts, cap MCPCTL_GATE_LLM_TIMEOUT_MS, default 8s) and pagination's smart index (llm/pagination.ts, MCPCTL_PAGINATION_LLM_TIMEOUT_MS, default 10s). read_prompts is LLM-free by design.

Note: the gate's prompt-ranking uses the heavy client provider's own model — it deliberately does not force the project's vLLM model onto it (doing so made every selection fail silently when the model wasn't anthropic-servable).

Instance health: live is not healthy

An MCP server answers tools/list from a static, in-process table. It costs a few milliseconds, needs no credentials, and reaches no upstream — so it stays green while the thing the server exists to talk to is unreachable. Treating that as a health signal is how mcpctl get instances showed eight healthy servers while the UniFi one had never once reached its controller.

So the probe reports two different passes:

Status	Probe	Means
healthy	readiness — tools/call on healthCheck.tool	The upstream answered. The server can do its job.
live	liveness — tools/list only	The process is up and speaks MCP. Its upstream is unverified.
degraded	either, failing	Failing, but under failureThreshold.
unhealthy	either, failing	Failed failureThreshold times in a row.

live is the default for any server with no healthCheck.tool. It is not a warning — it is an admission that nothing is watching that server's upstream.

Configure a readiness probe on every server. Pick a read-only tool that genuinely round-trips to the upstream, and verify it passes before configuring it — a probe naming a local-only tool (get_..._version) or a tool the server doesn't expose reproduces the same false green it was meant to remove.

mcpctl create server unifi-network --health-check-tool list_sites \
  --health-check-interval 60 --health-check-timeout 15 --force

or declaratively — healthCheck round-trips through get -o yaml | apply -f:

healthCheck:
  tool: list_sites
  arguments: {}
  intervalSeconds: 60
  timeoutSeconds: 15
  failureThreshold: 3

Omit tool to keep liveness while still tuning the timings.

Latency is the tell: a probe answering in single-digit milliseconds is reading a local table, not crossing a network. The UniFi probe went from 3ms (tools/list, lying) to 1847ms on its first real list_sites — login, TLS, controller round trip — and ~40ms once the session was warm.

Where a failing readiness probe usually points

Turning these probes on for the first time took the fleet from "8/8 healthy" to three genuine failures in under a minute. All three were network shape, not code — check these before suspecting the server:

1. Egress port. MCP server pods default to TCP 80/443 only (servers-allow-external-egress). Any upstream on another port — the UniFi controller on :8443 — times out on every call. Declare it in Pulumi's mcpctl.serverEgressTargets; don't widen the blanket rule.
2. Ingress hairpin. A co-located service reached over its public hostname goes out and back through the per-host Envoy L7 policy, which doesn't reliably carry the caller's identity and replies with a bare Access denied. Grafana 403'd on every call this way while the identical token succeeded from a laptop. The tell is the error shape: plain text, not the upstream's own JSON error. Use the ClusterIP (serverEgressTargets with namespace:).
3. Address reachability. A pod cannot reach a Tailscale 100.64.0.0/10 address. Config pointing at one connect-timeouts forever. Use LAN IPs. (This one turned out to be a retired service, which is its own kind of answer.)

Also check the dialect: UniFi's controller_type must be classic for a self-hosted controller (login /api/login, no /proxy/network prefix). unifi_os sends every request to a path that 404s.

LLM-essential operations — failover chain

Chat needs an LLM but not a specific one. Instead of failing when the pinned model is down, chat fails over across an ordered chain and reports which model actually answered.

• Chain: an Llm declares fallbacks in extraConfig.fallbacks: string[] (Llm names, in order). The dispatcher builds an ordered candidate list — the primary's pool, then each fallback's pool — and tries them in order.
• Fails over on real failures, not just transport: a non-2xx status (e.g. a drifted model's 400) or an empty/invalid completion now advances to the next candidate (chat.service.ts runOneInference). Streaming fails over pre-first-chunk.
• Transparency: ChatResult / the SSE final frame carry llm, model, and failedOver. The CLI prints model: <llm> (<model>) per turn, and ⚠ failed over → answered by <llm> (<model>) when a fallback was used.
• Exhaustion is clear: if every candidate fails, the error names the last model + upstream status/body (not "no choice").

Homelab chain: vllm-current (the served vLLM) → an anthropic-fallback server Llm as the always-up last resort. The chain is owned declaratively by Pulumi (kubernetes-deployment/deployments/mcpctl/llm-target.ts, fallbacks arg) so a pulumi up can't wipe it — mcpd replaces extraConfig on update.

A real last resort needs an independent cloud credential. LiteLLM only fronts the single local vLLM, so any local model shares the same GPU-box failure as the primary. anthropic-fallback points at api.anthropic.com and only serves once mcpd Secret anthropic-key holds a genuine sk-ant-api03 API key (from the Anthropic Console). A Claude subscription OAuth token (sk-ant-oat…, what claude setup-token mints) does not work: Anthropic gates those to the Claude Code client — probed directly they 404 on older model ids and 429 on current ones. mcpd's anthropic adapter does send OAuth tokens via Authorization: Bearer (so auth passes), but the gating is server-side and unavoidable. Until a real key is set, the fallback is wired-but-inert: an outage fails over to it and surfaces a clear anthropic-fallback (model) HTTP 4xx error rather than improving uptime.